The invention relates to a system and method for treating exhaust gas. More particularly, the invention relates to a system and method for treating exhaust gas of diesel engines.
In the restricted space available in exhaust systems of for example passenger cars, different converters may be arranged in a so-called closed coupled device. Such converters are, for example, an oxidizing converter to oxidize carbon monoxide and hydrocarbon and a selective catalytic reactor (SCR) to convert nitrogen oxides in the exhaust gas. In WO 2012/080585 an oxidation catalyst is arranged directly upstream of a diesel particle filter (DFP) and SCR combination. To support the catalytic reaction in the SCR, urea is injected into the limited space between the two catalysts. In order for the urea to mix with the exhaust gas, a mixing arrangement is provided between the two catalysts. However, such a mixing arrangement increases back pressure and a mixing path length before the mixture enters the SCR may not be increased. In addition, the temperature of the exhaust gas when urea is mixed into it has already decreased. Furthermore, the residence time of the urea and gas mixture is very low, particularly when exhaust gas flow is high.
It is also known to inject urea into a bypass parallel to an oxidation catalyst located upstream of a DPF and SCR combination to ensure a proper oxidation of hydrocarbon (HC) and carbon monoxide (CO). By this, exhaust gas taken from an upstream portion of the exhaust duct may directly be used for mixing with urea, which enables to operate a hydrolysis catalyst arranged in the bypass duct at a high temperature level. However, these systems require much space and do often not allow for a thorough mixing of urea with the exhaust gas before being introduced into the SCR. The portion of the exhaust gas passing through the bypass is not treated by the oxidation catalyst which may cause HC and CO emission failure during normalized driving cycles.
Therefore, there is a need for a system and method for treating exhaust gas taking care of the problems of known system and methods. Especially, there is a need for such a system and method which provides an efficient and a reliable exhaust gas treatment also in limited space conditions such as in closed coupled catalytic converter systems.